…his original line of argument for immaterialism, based on the subjectivity of colour, taste, and the other sensible qualities, was replaced by a simple, profound analysis of the meaning of “to be” or “to exist.” “To be,” said of the object, means to be perceived; “to be,” said of the subject, means to perceive.

…thus mere perceptions. The reality of the outside world is contingent on a knower. The 18th-century Anglo-Irish philosopher George Berkeley succinctly formulated his fundamental proposition thus: Esse est percipi (“To be is to be perceived”). In its more extreme forms, subjective idealism tends toward solipsism, which holds that I alone exist.

…the other furniture of the physical world, exist only insofar as they are in the mind of someone—i.e., only insofar as they are perceived. For any nonthinking being, esse est percipi (“to be is to be perceived”).

According to this argument, all of the qualities attributed to objects are sense qualities. Thus, hardness is the sensing of a resistance to a striking action, and heaviness is a sensation of muscular effort when holding the object in one’s hand, just as blueness is a quality of visual experience. But these qualities exist only while they are being perceived by some subject or spirit equipped…